1 Introduction
The XML Linking Language [XLink] defines Extensible
Markup Language (XML) 1.0 [XML] constructs to describe
links between resources. One of the stated requirements on XLink is to support HTML
[HTML 4.01] linking constructs in a generic way. The HTML BASE element is
one such construct which the XLink Working Group has considered. BASE allows authors
to explicitly specify a document's base URI for the purpose of resolving relative URIs
in links to external images, applets, form-processing programs, style sheets, and so
on.
This document describes a mechanism for providing base URI services to XLink, but
as a modular specification so that other XML applications benefiting from additional
control over relative URIs but not built upon XLink can also make use of it. The
syntax consists of a single XML attribute named xml:base.
The deployment of XML Base is through normative reference by new
specifications, for example XLink and the XML Infoset. Applications
and specifications built upon these new technologies will natively
support XML Base. The behavior of xml:base attributes
in applications based on specifications that do not have direct or
indirect normative reference to XML Base is undefined.
This specification does not attempt to specify which text strings in
a document are to be interpreted as URIs. That is the responsibility of
each XML vocabulary. The question addressed by this specification
is: given a relative URI in an XML document, what base URI is
it resolved against?
It is expected that a future RFC for XML Media Types will specify XML Base
as the mechanism for establishing base URIs in the media types it defines.
2 Terminology
[Definition: The key words
must, must not, required,
shall, shall not, should,
should not, recommended, may,
and optional in this specification are to be interpreted
as described in [RFC 2119].]
The terms base URI and relative URI
are used in this specification as they are defined in [RFC 3986].
3 xml:base Attribute
The attribute xml:base may be inserted in XML documents to
specify a base URI other than the base URI of the document or external
entity. The value of this attribute is interpreted as
a Legacy Extended IRI (LEIRI) as defined in
the W3C Note "Legacy extended IRIs for XML resource identification"
[LEIRI]
.
In namespace-aware XML processors, the "xml" prefix is bound to the namespace
name http://www.w3.org/XML/1998/namespace as described in Namespaces in XML
[XML Names]. Note that xml:base can be still used by
non-namespace-aware processors.
An example of xml:base in a simple document containing
XLinks follows. XLink normatively references XML Base for interpretation
of relative URI references in xlink:href attributes.
<?xml version="1.0"?>
<doc xml:base="http://example.org/today/"
xmlns:xlink="http://www.w3.org/1999/xlink">
<head>
<title>Virtual Library</title>
</head>
<body>
<paragraph>See <link xlink:type="simple" xlink:href="new.xml">what's
new</link>!</paragraph>
<paragraph>Check out the hot picks of the day!</paragraph>
<olist xml:base="/hotpicks/">
<item>
<link xlink:type="simple" xlink:href="pick1.xml">Hot Pick #1</link>
</item>
<item>
<link xlink:type="simple" xlink:href="pick2.xml">Hot Pick #2</link>
</item>
<item>
<link xlink:type="simple" xlink:href="pick3.xml">Hot Pick #3</link>
</item>
</olist>
</body>
</doc>The URIs in this example resolve to full URIs as follows:
"what's new" resolves to the URI "http://example.org/today/new.xml"
"Hot Pick #1" resolves to the URI "http://example.org/hotpicks/pick1.xml"
"Hot Pick #2" resolves to the URI "http://example.org/hotpicks/pick2.xml"
"Hot Pick #3" resolves to the URI "http://example.org/hotpicks/pick3.xml"
Note:
This specification does not give the xml:base attribute any special status as
far as XML validity is concerned. In a valid document the attribute must be
declared in the DTD, and similar considerations apply to other schema languages.
3.1 URI Reference Encoding and Escaping
The value of an xml:base attribute is a
Legacy Extended IRI (LEIRI)
and may contain characters not allowed in URIs.
(However, some characters allowed in LEIRIs are not legal XML characters, and
cannot therefore appear in xml:base values.)
In accordance with the principle
that percent-encoding must occur as late as possible in the processing chain,
applications which provide access to the base URI of an element
should
calculate and return the value without escaping.
In the example below, the base URI of element e2 should be
returned as "http://example.org/wine/rosé".
<?xml version="1.0"?>
<e1 xml:base="http://example.org/wine/">
<e2 xml:base="rosé"/>
</e1>
4 Resolving Relative URIs
4.1 Relation to RFC 3986
RFC 3986 [RFC 3986]
provides for base URI information to be
embedded within a document. The rules for determining the base URI can be
summarized as follows (highest priority to lowest):
The base URI is embedded in the document's content.
The base URI is that of the encapsulating entity (message,
document, or none).
The base URI is the URI used to retrieve the entity.
The base URI is defined by the context of the application.
Note:
The term "entity" in points #2 and #3 above uses the
RFC 3986
meaning of the term. Elsewhere in this document the term "entity"
is used in the XML sense.
This document specifies the details of rule #1 for embedding
base URI information in the specific case of XML documents.
4.2 Granularity of base URI information
Relative URIs appearing in an XML document are always resolved
relative to either an element, a document entity, or an external
entity. There is no provision for finer granularity, such as
per-attribute, per-character, or per-entity base information.
Neither internal entities, whether declared in the internal
subset or in an external DTD, nor freestanding text (text not
enclosed in an element) in an external entity, are considered
to set a base URI separate from the base URI in scope for the
entity reference.
The base URI of a document entity or an external entity is
determined by
RFC 3986
rules, namely, that the base URI is
the URI used to retrieve the document entity or external
entity.
The base URI of an element is:
the base URI specified by an xml:base attribute
on the element, if one exists, otherwise
the base URI of the element's parent element within the
document entity or external entity, if one exists, otherwise
the base URI of the document entity or external entity
containing the element.
Note:
It follows that the base URI specified by an xml:base attribute
is inherited by descendant elements within the same entity until another
element with an xml:base attribute is encountered.
The base URI of an element bearing an xml:base attribute with a value
that is not a valid Legacy Extended IRI is application dependent.
4.3 Matching URIs with base URIs
The base URI corresponding to a given relative URI appearing
in an XML document is determined as follows:
The base URI for a URI reference appearing in text content
is the base URI of the element containing the text.
The base URI for a URI reference appearing in an
xml:base attribute is the base URI of the parent
element of the element bearing the xml:base
attribute, if one exists within the document entity or
external entity, otherwise the base URI of the
document entity or external entity containing the element.
The base URI for a URI reference appearing in any other
attribute value, including default attribute values, is the
base URI of the element bearing the attribute.
The base URI for a URI reference appearing in the content
of a processing instruction is the base URI of the parent
element of the processing instruction, if one exists within
the document entity or external entity, otherwise the base
URI of the document entity or external entity containing
the processing instruction.
Note:
The presence of xml:base attributes might
lead to unexpected results in the case where the attribute value
is provided, not directly in the XML document entity, but via a
default attribute.
For instance, such a declaration in an external entity might
not be read by software which is based on
a non-validating XML processor. Defaulting attributes
through an external mechanism such as XML Schema may also lead to
unexpected results; even if a validating processor is used by the
application, the addition of defaulted attributes subsequent to
creation of the infoset can cause xml:base attributes to get out of
sync with the [base URI] infoset property. For these reasons,
xml:base values
should
be provided either directly in the XML document instance
or via default attributes declared
in the internal subset of the DTD.
4.4 Interpretation of same-document references
RFC 3986 defines certain relative URI references, in particular the
empty string and those of the form #fragment,
as same-document references. Dereferencing
of same-document references is handled specially. However, their use as
the value of an xml:base attribute does not involve
dereferencing, and XML Base processors should resolve them in
the usual way. In particular, xml:base="" does not
reset the base URI to that of the containing document.
Note:
Some existing processors do treat these xml:base
values as resetting the base URI to that of the containing document,
so the use of such values is strongly discouraged.
5 Conformance
An application conforms to XML Base if it calculates base URIs
in accordance with the conditions set forth in this specification.